Pressurized telephone cables are used to eliminate deleterious effects of moisture on the telephone lines and periodic checking of the gas pressure is necessary to detect leaks in the cable. For this purpose, pressure to resistance transducers are implanted in the cable at spaced points, each transducer being connected across the lines of a particular subscriber. Present telephone company arrangements for measuring the resistance of a particular transducer require disconnection of the telephone equipment and the application of a relatively high d.c. voltage across the subscriber's line, thereby resulting in the loss of service to the particular subscriber during the time the measurement is made. Also, it is necessary for the telephone company to keep a record of the various subscriber's lines across which the transducers are connected. This requires wiring changes in the central office whenever a subscriber's telephone number is changed.
Apparatus made in accordance with this invention measures the pressure in a selected region of the cable without interfering with or disrupting service to the subscriber's lines across which the pressure to resistance transducer is connected. The apparatus is compatible with all existing telephone equipment and with automatic data monitoring equipment.